1. Field of the Invention
The present invention relates to a reflective liquid crystal display device comprising a reflective pixel electrode, a method of manufacturing the reflective liquid crystal display device, and a liquid crystal display unit such as a reflective liquid crystal projector which displays an image through the use of the reflective liquid crystal display device.
2. Description of the Related Art
In recent years, with improvement in definition, miniaturization and brightness of projection displays, as display devices of the projection displays, reflective devices capable of reducing their size and displaying with high definition, and being expected to have high light utilization efficiency have become a focus of attention and have been put to practical use. A well-known reflective device is an active type reflective liquid crystal device in which a liquid crystal is injected between a pair of substrates facing each other. In this case, as the pair of substrates, a transparent electrode substrate formed through laminating a transparent electrode on a glass substrate, and a drive substrate using a silicon (Si) substrate including, for example, a CMOS (Complementary-Metal Oxide Semiconductor) type semiconductor circuit are used. On the drive substrate, a reflective pixel electrode of metal for reflecting light and applying a voltage to the liquid crystal is disposed so as to form a pixel electrode substrate. The reflective pixel electrode is made of a metal material including aluminum as a main component, which is generally used in a LSI (Large Scale Integrated) process.
In such a reflective liquid crystal display device, when a voltage is applied to the transparent electrode disposed on the transparent electrode substrate and the reflective pixel electrode disposed on the drive substrate, a voltage is applied to the liquid crystal. At this time, a change in optical properties of the liquid crystal occurs depending upon a potential difference between these electrodes, thereby the liquid crystal modulates incident light. The reflective liquid crystal display device can display a gray-scale image by the modulation of the light.
In such a reflective liquid crystal display device, specifically an active type reflective liquid crystal display device into which a vertically aligned liquid crystal is injected has become a focus of attention as a projection device in recent years, because the active type reflective liquid crystal display device has high contrast and high response speed. Herein, “a vertically aligned liquid crystal material” means a liquid crystal material with negative dielectric anisotropy (a difference Δε (=ε(∥)−ε(⊥)) between a dielectric constant ε(∥) parallel to a long axis of liquid crystal molecules and a dielectric constant ε(⊥) perpendicular to the long axis of the liquid crystal molecules is negative), and in the vertical aligned liquid crystal material, when an applied voltage is zero, the liquid crystal molecules are aligned in a substantially vertical direction with respect to a substrate surface, thereby the active type reflective liquid crystal display device operates in a normally black mode.
In the vertical aligned liquid crystal, when an applied voltage is zero, the long axis of the liquid crystal molecules is aligned in a substantially vertical direction with respect to each substrate surface, and when a voltage is applied, the long axis is aligned in an in-plane direction, thereby the transmittance of the vertically aligned liquid crystal is changed. The direction is a diagonal direction of the pixel electrode (that is, a 45° direction). If the liquid crystal molecules are not aligned in the same direction during drive, the contrast becomes uneven, so in order to prevent this, it is required to align the liquid crystal molecules at a very small pretilt angle in a predetermined direction in advance, and then vertically align them. When the pretilt angle is too large, vertical alignment is degraded, and a black level is increased, thereby the contrast declines. Therefore, in general, the pretilt angle is controlled within a range from 1° to 5° in a diagonal direction of the pixel electrode with respect to the direction of the normal to the substrate surface.
There are two methods of aligning the vertically aligned liquid crystal material, that is, a method of controlling alignment by using an organic alignment film typified by polyimide and rubbing it, and a method of controlling alignment through oblique evaporation by using an inorganic alignment film typified by silicon oxide. Nowadays, in order to achieve higher brightness of a projector, there is a tendency that the power of a lamp is increased to irradiate a display panel with light with very high intensity. Therefore, a problem that the organic alignment film in the former method is degraded due to the light arises.
On the other hand, an obliquely evaporated film of silicon oxide in the latter method is an inorganic material, so unlike polyimide, no degradation in the material due to light occurs, and higher reliability can be achieved. Therefore, the obliquely evaporated film becomes a focus of attention. In the case where the alignment film is the obliquely evaporated film of silicon oxide, the incident angle of evaporation particles to a substrate is changed during oblique evaporation to control the pretilt angle. In general, a practical incident angle is within a range from approximately 45° to 65° with respect to the direction of the normal to a substrate.
A technique of related art of forming the alignment film through oblique evaporation is described in, for example, Japanese Unexamined Patent Application Publication No. 2001-5003.
However, in general, it is difficult to control the alignment of the vertically aligned liquid crystal material. In the case where there is an uneven structure on the drive substrate by the reflective pixel electrode or a groove between pixel electrodes, an alignment defect occurs around the pixel electrode due to the uneven structure. The alignment defect causes a decline in uniformity of characteristics in a display surface, an increase in black level (a phenomenon in which a black portion of an image does not appear black but gray), degradation in image quality due to disclination. In particular, in a reflective liquid crystal display device using a silicon drive device, a pixel pitch is as small as 10 μm or less in general, so compared to a large direct-view-type liquid crystal device with a pixel pitch of a few tens of μm or more, a defective region around a pixel easily exerts an influence upon image quality, and unlike a transmissive liquid crystal display device, the defective region cannot be covered with a black matrix, so a basic practical requirement for the reflective liquid crystal display device is that a misalignment region must be reduced to a minimum or completely eliminated.
A specific problem of the reflective liquid crystal display device of the related art due to pixel electrode structure will be described below. As shown in FIGS. 1A and 1B, reflective pixel electrodes 111 are disposed on a silicon drive substrate 110 in a matrix. The size and the shape of each of the reflective pixel electrodes 111 is, for example, a square 8.4 μm on a side. In order to prevent an electrical short circuit between adjacent pixels, the reflective pixel electrodes 111 are disposed so as to have an inter-pixel spacing W1 with a predetermined distance. When the inter-pixel spacing W1 is, for example, 0.6 μm, a pixel pitch W2 is 9 μm. In general, the pixel pitch W2 is within a range of approximately 7 μm to 15 μm, and the inter-pixel spacing W1 is within a range of approximately 0.3 μm to 0.7 μm. Further, the thickness of the pixel electrode is within a range of approximately 150 nm to 250 nm.
As each of the reflective pixel electrodes 111 has such a shape, a portion with a groove-like shape (hereinafter referred to as inter-pixel groove) is always formed between adjacent pixel electrodes. The inter-pixel groove has, for example, an aspect ratio of 600 nm wide to 150 nm high in a sectional surface shown in FIG. 1B.
FIGS. 2 and 3 schematically show a state where an alignment film 112 of silicon oxide is formed on the pixel structure shown in FIGS. 1A and 1B through oblique evaporation and a state of the alignment of a vertically aligned liquid crystal 113 by the alignment film 112. In FIGS. 2 and 3, an arrow 130 shows an evaporation direction. The alignment film 112 is obliquely evaporated on a substrate at, for example, an incident angle θ of 55° (refer to FIG. 2) with respect to the direction of the normal to a substrate surface from a diagonal direction of the reflective pixel electrodes 111 (refer to FIG. 3).
In the case where such oblique evaporation is performed, as shown in FIG. 2, an area around a side surface of the reflective pixel electrode 111 opposite to the incident direction (around a region 121 in FIG. 2) is shaded with the reflective pixel electrode 111, so the alignment film 112 is not evaporated and formed on the area. On the other hand, the alignment film 112 is formed in the shape of the letter L around a side surface on the other side, as shown in FIG. 2. Thus, the region 121 where no alignment film 112 is formed exists on a bottom surface of the inter-pixel groove and the side surface of the reflective pixel electrode 111.
The alignment direction of a pretilt is a diagonal direction of pixels, and FIG. 3 shows a schematic plan view of a region where the alignment film 112 is formed and the region 121 where the alignment film 112 is not formed in this case. When the thickness of the reflective pixel electrode 111 is increased, and the inter-pixel spacing W1 is reduced, no film is evaporated on the bottom surface of the inter-pixel groove, and the film is formed only on one side surface of the inter-pixel groove. In a typical method of forming an alignment film, it is inevitable that the film structures of both side surfaces of the inter-pixel groove become asymmetric.
Thus, as a region where the alignment film 112 is not formed specifically on the bottom surface of the inter-pixel groove exists, the alignment of the liquid crystal 113 cannot be controlled in the region, thereby the liquid crystal 113 goes out of alignment, and problems such as degradation in image quality such as nonuniform alignment and a decline in reliability arise. In other words, as shown in FIG. 2, the alignment film 112 is formed on a surface of the reflective pixel electrode 111, thereby the long axis of liquid crystal molecules is uniformly aligned in a pretilt direction in a good state in general. On the other hand, specifically the region 121 where the alignment film 112 is not formed is produced in a portion of the bottom surface of the inter-pixel groove, so a force to vertically align the liquid crystal molecules does not work, thereby a nonuniform alignment region 120 is produced. The nonuniform alignment region 120 exerts an influence upon an area around the pixel electrode, thereby resulting in a state where liquid crystal molecules on the surface of the pixel electrode are vertically aligned, but liquid crystal molecules in a region from an area around the pixel electrode to the inter-pixel groove are nonuniformly aligned. Thereby, nonuniform alignment occurs in the region from the area around the pixel electrode to the inter-pixel groove, thereby degradation in image quality is induced. The evaporation angle is generally selected within a range of 45° to 65° with respect to the direction of the normal to the substrate; however, the deeper the inter-pixel groove is, the larger the region where the alignment film 112 is not formed in the bottom surface of the inter-pixel groove becomes, so the evaporation angle has a large influence. The above phenomenon specifically occurs in the case where an obliquely evaporated film of an inorganic material such as silicon oxide is used as the alignment film 112.
On the other hand, in an organic alignment film such as polyimide, the above problems which arise because the alignment film 112 is not formed do not occur. It is because the organic alignment film is formed through coating the whole surface of the pixel substrate with a material in a solvent form by a technique such as spin coating, so the inter-pixel groove is overcoated with the material on average.
A technique proposed in Japanese Unexamined Patent Application Publication No. 2001-5003 is that at first, oblique evaporation is performed at an angle of 70° from the normal to a substrate surface along a side of a pixel electrode to form a first alignment film on a bottom surface portion of an inter-pixel groove along the side of the pixel electrode, and then the substrate is rotated by 90° in a plane, and a second alignment film is formed on a bottom surface portion of the inter-pixel groove along the other side of the pixel electrode through the same oblique evaporation.
According to the technique in Japanese Unexamined Patent Application Publication No. 2001-5003, an alignment film is surely formed on the bottom surface portion of the inter-pixel groove. However, as described above, in the technique, the alignment film cannot be formed on the whole inter-pixel groove to be exact unless evaporation is performed along a side of the pixel electrode, and the substrate is rotated by 90° in a plane to perform evaporation again. However, even if the first alignment film and the second alignment film are formed from different directions by 90° in a plane, a region where no film is formed exists on a side surface of the inter-pixel groove, so asymmetry in the side surfaces of the inter-pixel groove cannot be solved. Therefore, the above burn-in due to ions occurs.
Moreover, in general, in the reflective liquid crystal display device, as a polarization splitting device, a PBS (Polarization Beam Splitter) is used. When polarization is split in a cross Nicol arrangement by the PBS, the alignment direction of a vertical liquid crystal which can obtain the highest transmittance is diagonal to a pixel, that is, a 45° direction. Therefore, in the alignment along the side of the pixel in Japanese Unexamined Patent Application Publication No. 2001-5003, a polarization splitting optical system using the PBS cannot be used in the reflective liquid crystal display device, and the reflective liquid crystal display device has little practicability specifically as a projection display unit. In order to avoid the problem, when the second alignment film is formed on a direction diagonal to the pixel, in principle, even if the first alignment film is formed from any direction, a region which cannot be fully covered exists in the inter-pixel groove, so effects of the technique in Japanese Unexamined Patent Application Publication No. 2001-5003 are not exerted. Therefore, the technique is far from practically effective.